1 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Programming with OpenGL Part 4: Color and Attributes Isaac Gang University of Mary Hardin-Baylor

2 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Objectives Expanding primitive set Adding color Vertex attributes Uniform variables

3 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 OpenGL Primitives GL_TRIANGLE_STRIP GL_TRIANGLE_FAN GL_POINTS GL_LINES GL_LINE_LOOP GL_LINE_STRIP GL_TRIANGLES

4 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Polygon Issues OpenGL will only display triangles ­Simple: edges cannot cross ­Convex: All points on line segment between two points in a polygon are also in the polygon ­Flat: all vertices are in the same plane Application program must tessellate a polygon into triangles (triangulation) OpenGL 4.1 contains a tessellator nonsimple polygon nonconvex polygon

Polygon Testing Conceptually simple to test for simplicity and convexity Time consuming Earlier versions assumed both and left testing to the application Present version only renders triangles Need algorithm to triangulate an arbitrary polygon 5 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012

Good and Bad Triangles Long thin triangles render badly Equilateral triangles render well Maximize minimum angle Delaunay triangulation for unstructured points 6 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012

Triangularization Convex polygon Start with abc, remove b, then acd, …. 7 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 a c b d

Non-convex (concave) 8 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012

Recursive Division Find leftmost vertex and split 9 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012

10 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Attributes Attributes determine the appearance of objects ­Color (points, lines, polygons) ­Size and width (points, lines) ­Stipple pattern (lines, polygons) ­Polygon mode Display as filled: solid color or stipple pattern Display edges Display vertices Only a few (glPointSize) are supported by OpenGL functions

11 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 RGB color Each color component is stored separately in the frame buffer Usually 8 bits per component in buffer Color values can range from 0.0 (none) to 1.0 (all) using floats or over the range from 0 to 255 using unsigned bytels

12 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Indexed Color Colors are indices into tables of RGB values Requires less memory ­indices usually 8 bits ­not as important now Memory inexpensive Need more colors for shading

13 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012 Smooth Color Default is smooth shading ­OpenGL interpolates vertex colors across visible polygons Alternative is flat shading ­Color of first vertex determines fill color ­Handle in shader

Setting Colors Colors are ultimately set in the fragment shader but can be determined in either shader or in the application Application color: pass to vertex shader as a uniform variable (next lecture) or as a vertex attribute Vertex shader color: pass to fragment shader as varying variable (next lecture) Fragment color: can alter via shader code 14 E. Angel and D. Shreiner: Interactive Computer Graphics 6E © Addison-Wesley 2012